Learning Objective: Recognize the importance of oxygen to include types, characteristics, and the effects of a lack of oxygen.

No one can live without sufficient quantities of food, water, and oxygen. Of the three, oxygen is by far the most urgently needed. If necessary, a well-nourished person can go without food for many days or weeks, living on what is stored in the body. The need for water is more immediate but still will not become critical for several days. The supply of oxygen in the body is limited to a few minutes. When that supply is exhausted, death is inevitable. Oxygen starvation affects a pilot or air-crewman in much the same way that it affects an aircraft engine. Both the body and the engine require oxygen for the burning of fuel. An engine designed for low-altitude operation loses power and performs poorly at high altitudes. High-altitude operation demands a means of supplying air at higher pressure to give the engine enough oxygen for the combustion of fuel. A super-charger or compressor satisfies the engines demands. What about the demands of the human body? The combustion of fuel in the human body is the source of energy for everything the aviator is required to do with muscles, eyes, and brain. As the aircraft climbs, the amount of oxygen per unit of volume of air decreases, and the aviator's oxygen intake is reduced. Unless the aviator breathes additional oxygen, the eyes, brain, and muscles begin to fail. The body is designed for low-altitude operation and will not give satisfactory performance unless it is supplied the full amount of oxygen that it requires. Like the engine, the body requires a means of having this oxygen supplied to it in greater amounts or under greater pressure. This need is satisfied by use of supplemental oxygen supplied directly to the respiratory system through an oxygen mask, and by pressurizing the aircraft to a pressure equivalent to that at normal safe-breathing altitudes, or both. For purposes of illustration, an aviator's lungs are like a bag of air since the air in the lungs behaves in the same way. If an open bag is placed in an aircraft at sea level, air will escape from it continuously as the aircraft ascends. The air pressure at 18,000 feet is only half that at sea level; therefore, at 18,000 feet the bag will be subjected to only half the atmospheric pressure it was subjected to at sea level. For this reason, it will contain only half the oxygen molecules it had when on" the ground. In like fashion, an aviator's lungs contain less and less air as he/ she ascends and correspondingly less oxygen. Thus, the use of supplemental oxygen is necessary on high-altitude flights.

Up to approximately 35,000 feet, an aviator can keep sufficient oxygen in his/ her lungs to permit normal activity by use of oxygen equipment that supplies oxygen upon demand (inhalation). The oxygen received by the body on each inhalation is diluted with decreasing amounts of air up to approximately 33,000 feet. Above 33,000 feet and up to approximately 35,000 feet, this equipment provides 100-percent oxygen. At approximately 35,000 feet, inhalation through the DEMAND oxygen system alone will NOT provide enough oxygen. Above 35,000 feet and up to about 43,000 feet, normal activity is only possible by use of PRESSURE DEMAND equipment. This equipment consists of a "supercharger" arrangement by which oxygen is supplied to the mask under a pressure slightly higher than that of the surrounding atmosphere. Upon inhalation, oxygen is forced (pressured) into the mask by the system. Upon exhalation the oxygen pressure is shut off automatically so that carbon dioxide can be expelled from the mask. Above 43,000 feet, the only adequate provision for the safety of the aviator is pressurization of the entire body.

TYPES OF OXYGEN 

Aviators breathing oxygen (MIL-0-2721OD) is supplied in two types- type I and type II. Type I is gaseous oxygen and type II is liquid oxygen. Oxygen procured under this specification is required to be 99.5 percent pure. The water vapor content must not be more than 0.02 milligrams per liter when tested at 21.1 C (70 F) and at sea-level pressure. Technical oxygen, both gaseous and liquid, is procured under specification BB-O-925A. The moisture content of technical oxygen is not as rigidly controlled as is breathing oxygen; therefore, the technical grade should never be used in aircraft oxygen systems. The extremely low moisture content required of breathing oxygen is not to avoid physical injury to the body, but to ensure proper operation of the oxygen system. Air containing a high percentage of moisture can be breathed in-definitely without any serious ill effects. The moisture affects the aircraft oxygen system in the small orifices and passages in the regulator. Freezing temperatures can clog the system with ice and prevent oxygen from reaching the user. Therefore, extreme precautions must be taken to safeguard against the hazards of water vapor in oxygen systems.